I’ve done a quick rough replica of your object, and removed the bottom face.
Do you want to rotate/move just that one edge, or the face?
Or all of them?
Is this what you’re trying to achieve??..
After removing the bottom face, in Edit Mode > Edge Select.
Select all bottom edges and activate the Scale Tool.
Either use the gizmo or type in the X,Y, scale proportions in the Resize popup, 0.9 for both X and Y appears to create that 90° angle.
(This technique will still work with the bottom face intact)
Hope this helps somehow
This shows the bottom face intact with Face Normals at 90°